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Low voltage virtual ground

Gary Richardson
Intellectual 400 points
Other Parts Discussed in Thread: LMV721, LM8272, LM7321, LM8262
I would like to use a LMV721 (or other similar opAmp) to establish a 1.65V virtual ground; the VCC is 3.3 volts. I would like the impedance of the virtual ground to remain as low as possible to at least 10 MHz and higher if possible. What is the largest value of capacitance that can appear on the output of the '721?
 
My virtual ground would not create any DC load on the amplifier. Do I need a DC load? If so, how large?
  • 0
    TI__Guru**** 150626 points

    Gary,

    The data sheet pages 8 to 11 shows that driving a capacitor is not what the LMV721 does well.
    What is the main reason not to use no op-amp, just two equal resistors and capacitor(s)?

  • 0
    TI__Guru 53746 points

    Hi Gary,

    As Ron said, the LMV721 (and about 95% of most op-amps) do not like capacitors hung directly on their output - particularly in a "follwer" configuration.

    How much current do you need? It is possible to add a series resistor inside the feedback loop to add capacitive isolation but preserve DC balance - but you add some output impedance.

    Remember that the op-amps output impedance rises as frequency goes up - so it is not always "zero" - especially at 10MHz. So the capacitors should provide the low impedance down as low a frequency as possible. Use switcher grade tantalums for the large caps and SMT ceramics for the small - not cheap aluminum electrolytics.

    We do make high capacitive load drive op-amps, that were originally designed to drive LCD screen VCOM lines or other high capcitence loads. The main difference with these op-amps is their lower output impedance and internal compensation to keep them stable. This is why they have higher supply currents than similar BW amps.

    See the LM7321/2 or LM8262 or LM8272 - these are the "unlimited" cap load drive devices.

    BTW: I do not like the "unlimited cap load" designation. There is a limit, usually the output current available to charge/discharge the capacitor. If your application is mostly DC with small transients, then the above devices should do fine.

    Regards,